1. Field of the Invention
This invention relates to improvements in pump chambers of the type used in beverage makers wherein water in the chamber is heated to boiling to force the water up a supply tube into an infusion basket which holds the beverage ingredients or other material to be infused. More particularly, the invention relates to a chamber structure which minimizes the tendency for stable mineral deposits to form on the chamber wall.
2. Description of the Prior Art
Pump chambers of the type in question have long been used in percolators and automatic coffee makers. In general, the heated wall contacting the liquid within the chamber (or "well" as it will be termed hereinafter for simplicity, although the chamber can be of any desired shape--including long, relatively shallow chambers) has been smooth, that is, formed by an element of revolution. Where the water supply has had a relatively high mineral content, so-called "hard" water, it has been quite common for the repeated heating cycles attendant upon brewing of the beverages to cause deposition of successive thin layers of minerals (termed "waterstone", hereinafter for convenience) which cling to the smooth wall and effectively insulate it. With respect to the mineral content of water, it may be pointed out that the problem is a general one--even in the 100 largest cities in the U.S. there are some who must use water with hardness of about 180 parts per million or higher (U.S. Geological Survey, 1962 per Todd, K. K., Ed. The Water Encyclopedia, Water Information Center, Huntington, N.Y., 1970).
In the past, the above-described problem has been solved by requiring the user to scour the interior of the pump well or treat it with vinegar (acid) periodically (monthly, for instance, if the pump well is in a coffee pot used daily) but this is a nuisance and oftentimes overlooked. As an alternative, it has been suggested in prior art U.S. Pat. No. 3,856,435 to solve the problem by making the pump in two sections--an upper housing and a lower housing (i.e. the well proper)--using materials having different thermal properties, e.g., the upper housing being made of a material of lower thermal conductivity than that of the material used for the well. Such requirements for different materials make the device generally more costly. In addition to the foregoing, we have also investigated various other approaches for solving the problem of mineral buildup. In one approach, coatings of materials which enhance release of the deposited layers (e.g. materials such as silicone elastomers) were applied to the walls, but these materials were themselves insulators, so the coatings increased the temperature at the exterior just as the mineral deposits did. In view of the insulative aspect of the solid release material, another approach involved applying to the wall a very thin film of an edible oil, but improvement was only transitory, the film apparently wearing off such that once it was gone, pump temperature began to climb at a rate similar to that for a standard pump, i.e. one without the surface treatment. Yet another approach involved polishing the wall surface but that too was relatively ineffective, as will be discussed subsequently. Likewise, it was thought that mechanical agitation might be beneficial, so a marble was placed in the well and allowed to "rattle" during cycling to chip away the waterstone. No effect on rate of waterstone buildup was noted. Also, use of a seeding technique was a possibility suggested in the literature on saline water conversion where a decrease in mineral deposition was noted when a slurry of particles of like nature to the mineral deposits was entrained in the water flow. The theory was that the lowest energy state would be achieved by deposition on a like material. Accordingly, we placed calcium carbonate chips in the well, but again no beneficial effect was noted. Lastly, use of pins (wires) inserted in the base of the well was investigated, but this achieved little improvement in reducing waterstone buildup in the major area of deposit, i.e. the vertical walls.
From the foregoing, it is seen that there is need of further improvement in pump wells to make the performance of such wells less sensitive to the presence in the water of minerals which tend to deposit within the well and interfere, ultimately, with proper operation of the device utilizing the well.